1. Field of the Invention
The invention is concerned with optical focusing systems.
2. Description of the Prior Art
Optical focusing systems having variable focal length are widely used in optical imaging systems such as cameras, telescopes, binoculars, and projectors. Such systems typically comprise one or several glass or plastic lenses whose position is varied by a mechanical drive so as to effect a continuous change of focal length. Also, variation of focal length may be effected in lens systems in which the relative position of individual lenses is adjustable.
A variable focusing effect has also been achieved in a system in which light scattering particles are suspended in a fluid host medium and suitably arranged in such medium by means of an applied electric or magnetic field. Specifically, U.S. Pat. No. 3,531,185, "Liquid Lens Optical Guide Employing Neutral Particles Supported in the Liquid," S. J. Buchsbaum et al., discloses a light guide in which light undergoes repeated focusing as its travels in a fluid medium doped with light scattering particles, greater or lesser focal length being achieved depending on the strength of the applied field.
Systems based on doped fluids have also been proposed for purposes other than optical focusing. For example, U.S. Pat. No. 3,512,876, "Dipolar Electro-Optic Structures," A. M. Marks, discloses the use of a doped fluid system for electrically controlled optical filtering and shuttering. Yet another type of electro-optical device is disclosed in U.S. Pat. No. 3,374,358, "Light Display Device Employing Angularly Selective Polarizer," H. Majima. This latter device produces x-y displays similar to those obtainable by means of a cathode ray oscilloscope and is based on the rotation of the direction of polarization of polarized light in an optically active medium by an amount which depends on the strength of a voltage applied across the medium.
More recently, devices have been proposed which are based on electro-optical and magneto-optical effects in so-called liquid crystal materials, i.e., materials which are characterized in that they have at least one semi-ordered, mesomorphic phase in addition to a solid phase and an isotropic liquid phase. Well known mesomorphic phases are the smectic, nematic, and cholesteric phases, the nematic phase being of particular importance in the context of the present application. Molecules of nematic liquid crystal materials typically are rod-shaped or perhaps disk-shaped. In the former case the average direction of the long axes of molecules in a small region is designated the director of that region, in the latter case it is the direction, perpendicular to the disks which is used to define the director. The nematic phase of a nematic liquid crystal material is characterized in that directors of the material are aligned in a preferred direction.
R. A. Soref, "Electronically Scanned Analog Liquid Crystal Displays," Applied Optics Vol. 9, pages 1323-1329 (June 1970) discloses an optical display device comprising a thin layer of liquid crystal material sandwiched between optically transparent, electrically conducting electrodes. In response to an applied voltage, portions of the liquid crystal layer change from a transparent state to a light scattering state, thereby changing in visual appearance. A device having electrodes which may be opaque is disclosed in U.S. Pat. No. 3,674,342, "Liquid Crystal Display Device Including Side-by-Side Electrodes on a Common Substrate," J. A. Castellano et al, which appears to be particularly suitable for the display of line figures.
The use of liquid crystal materials has also been proposed for purposes other than optical display. For example, U.S. Pat. No. 3,741,629, "Electronically Variable Iris or Stop Mechanism, " F. J. Kahn, discloses the use of a layer of liquid crystal material in combination with annular electrodes to serve as an electrically controlled iris.